This invention relates to a novel pavement marking sheet material of an improved durability.
Various sheet materials have heretofore been known as ones adhered to roadway for the pavement marking purpose.
One type of such sheet materials is of a construction in which a microsphere support layer of a thin film resin in which glass microspheres are partially embedded is adhered to the surface of a base layer. This type of sheet material is disclosed, for example, in the specifications of U.S. Pat. Nos. 4,248,932, 4,282,281, 4,299,874, 4,117,192, 2,440,584, 3,915,771 and 3,764,455. These sheet materials have two characteristic features common through all of these specifications: on is that the lower hemispheres of the microspheres are supported only by the support layer and they never reach the inside of the base layer. The other is that the upper hemispheres of the microspheres protruding above the support layer and performing the function of reflecting light are exposed to the air.
The above-mentioned first characteristic that the microspheres do not reach the inside of the base layer is an unavoidable result of the prior art method of manufacturing the sheet material according to which the microsphere support layer having the lower hemispheres of the microspheres embedded previously is separately made and this support layer is adhered to the base layer.
The second characteristic that the upper hemispheres of the microspheres are exposed to the air is also an unavoidable construction for supporting the microspheres only by the support layer containing the coloring agent.
Pavement marking sheet material must be colored in colors which are distinguishable for passers-by. Since the base layer of the sheet needs to be thicker than the support layer for imparting it with durability, the addition of a coloring agent to the base sheet entails larger consumption of the coloring agent and therefore is uneconomical. For this reason, a substantial amount of the coloring agent must be added to the support layer with a result that the support layer becomes opaque by the coloring.
If the glass microspheres are entirely embedded in this opaque support layer, the microspheres apparently will not be able to perform the necessary light reflecting function. Thus, the upper hemispheres of the glass microspheres are exposed to the air and no further consideration has been given to this question.
This construction, however, allows the glass microspheres dispersed on the surface of the sheet and partially embedded in the support layer to contact wheels of running traffic directly. Consequently, this construction inevitably has the tendency that the glass microspheres come off from the support layer.
Since the upper hemispheres of the microspheres must be substantially exposed for enabling the microspheres to perform the light reflecting function and the portions of the microspheres embedded in the support layer are relatively small the support layer naturally becomes thin with a result that the above described tendency cannot be sufficiently prevented no matter what excellent adhesive to the microspheres may be chosen as the material of the support layer.
One of the above listed prior arts teaches that in case the support layer is worn by contact with wheeled traffic and the microspheres thereby come off, other microspheres embedded in the base layer come to be exposed so that no serious problem will arise. It will not be desirable, however, that the glass microspheres on the surface readily come off from the support layer.